How to Unlock Joker Guide
In order to unlock the Joker monster in Monster Rancher 2 you'll first need to raise your Trainer's Rank to 4th or better. You will also need to go on the Kawrea adventure at least two times, the first time you'll need to trigger the scene with the Phoenix flying out of a volcano. The second time you come here, after triggering that scene, you'll be able to explore much more of the area.
Once you've unlocked the entire Kawrea Expedition area, you'll be able to find the Burned Altar. This building is a bit confusing to get to since there's two paths in the far end of the adventure area; one leads to a fiery tower which is as far back as you can go and the second path leads to the Burned Altar. The path closest to the place you make camp for the expedition is the one that leads to the Burned Altar.
Inside the Burned Altar you'll find the Joker's Mask, which is the item you'll want to use as a secret seasoning to unlock the Joker monster. This item has an extremely low drop rate from inside the Burned Altar so you might have to make multiple trips to this adventure to find it.
Though once you've uncovered the mask all you need to do is watch the scenes that follow back at the Ranch, then take it to the Lab and you'll have your monster.